The Javadoc for ResponseBuilder.tag() says that "The JAX-RS runtime will quote the supplied value when creating the header".
RESTEasy does not quote the supplied value when creating the header, though.
Here are the response headers obtained with RESTEasy and Jersey:
RESTEasy
--------
HTTP/1.1 200 OK
...
ETag: 123
...
Jersey
------
HTTP/1.1 200 OK
...
ETag: "123"
...